Hi there, Ryan! Thanks for getting in touch and checking out the plugins. 🙂
Good question. The Events Calendar is a free plugin that you can grab on WordPress.
The Evenbrite Tickets add-on is a separate plugin that does require a license.
The Events Calendar PRO is not necessary to run the Eventbrite Tickets add-on. Eventbrite Tickets will run as billed with the free version of The Events Calendar. However, if you do want The Events Calendar PRO, then it will require its own license as well.
Does this help answer your question? Please let me know.
Cheers!
Geoff